Republican debate highlights and analysis: Fiery faceoff on Trump, Ukraine and more

One challenger to Trump has surged ahead — in ad buys, at least. Scott announced an $8 million ad buy this month. The bulk of that, $6.6 million, will go toward TV advertisements in early primary states Iowa and New Hampshire, according to the Washington Post. That’s on top of the $40 million ad buy planned by a super PAC supporting Scott that was announced last month.

Trump and DeSantis, the two front-runners, had dominated in early ad spending. But challengers like Scott have stepped up as they’ve tried to qualify for debates and raise name recognition in early voting states. Super PACs supporting Haley and Burgum have also increased spending recently. Stand for America Fund, Inc., which is aligned with Haley, spent $2.3 million on ads so far in August, while Best of America, which backs Burgum, has spent $2.1 million, according to reporting from NBC News.

Meanwhile, Trump-supporting PACs have spent millions on the former president’s legal fees as the number of indictments filed against him has risen to four. That will leave less money for spending on ads going forward, but Trump remains the top campaign fundraiser.

-Analysis by Monica Potts of FiveThirtyEight

In 2016, Trump was the ultimate outsider candidate — he didn’t receive a single endorsement from a sitting senator, representative or governor until after he had already won New Hampshire, South Carolina and Nevada. This year, though, he already has a ton of elite support. According to the FiveThirtyEight endorsement tracker, he has 343 endorsement points — nearly eight times as many as DeSantis, who is in second place.

The 2016 race notwithstanding, endorsements have historically been very predictive of who eventually wins a party’s nomination. The candidate with the most FiveThirtyEight endorsement points on the day before the Iowa caucuses has won 11 of the last 17 Republican and Democratic primary fights without an incumbent president. And when that candidate has a dominant endorsement lead, they win even more often: Endorsement leaders who had collected more than 15 percent of the estimated available endorsement points before Iowa won nine times out of 10. And Trump already has 17 percent of all available endorsement points with five months left until Iowa.

-Analysis by Nathaniel Rakich of FiveThirtyEight

Recent election cycles have shown how debates can substantially alter a candidate’s trajectory. The debates won’t always do this, of course, and early primary polls remain relatively predictive of who will end up winning a party’s nomination. However, a candidate can earn a real polling bounce from a debate — or fall into a valley.

Primary polls are inherently more volatile than general election polls. In a general election, most voters already lean toward one party, so only a small part of the electorate is likely to shift its views over the course of a campaign. But in a primary, voters in one party may be open to multiple candidates because most contenders will share similar views on a number of issues. As a result, exposure to an unfamiliar candidate could prompt voters to begin considering that contender, while a strong or weak performance by a candidate they do know could make them look elsewhere.

For example, early in the 2012 cycle, Texas Gov. Rick Perry appeared to be now-Sen. Mitt Romney’s most serious opponent for the Republican nomination. But in a September 2011 debate, Perry defended a Texas policy that gave undocumented immigrants access to in-state college tuition. The backlash from the GOP base came swiftly, as Perry fell out of contention in the polls, well before a later debate in which he — oops — forgot the name of the third of three federal agencies that he aimed to dismantle as president. Four years later, former tech executive Carly Fiorina had very little support early in the 2016 Republican race, but in August 2015 she stood out in an “undercard” debate. Her polling numbers improved and she made the main stage in subsequent debates, giving her a platform that eventually led Texas Sen. Ted Cruz to make her his vice presidential pick in a last-ditch attempt to rally support against Trump in the 2016 primary.

-Analysis by Geoffrey Skelley of FiveThirtyEight

Trump teased for months whether or not to attend the first GOP primary debate, citing along the way his yawning leads in early national and statewide polls.

"Why would I allow people at 1 or 2% and 0% to be hitting me with questions all night?" he said in an interview in June with Fox News anchor Bret Baier.

When he ultimately decided to not appear on stage on Wednesday, it was precisely that lead he cited.

"The public knows who I am & what a successful Presidency I had," Trump wrote on social media, adding, "I WILL THEREFORE NOT BE DOING THE DEBATES!"

It also appears unlikely that Trump will attend the second debate hosted next month by The Ronald Reagan Presidential Foundation & Institute, the administration of which Trump has feuded with in the past.

"He's not going to be in California," senior Trump campaign adviser Chris LaCivita said on the conservative "Ruthless" podcast Wednesday. "Well, first of all, with President Trump, you know, he could change his mind at any moment. Right. But I think he said the other day, he should take his history for face value when he said debates, right? And you should just assume that he won't until he does."

-ABC News' Tal Axelrod